Poly(lactide) (PLA) is one of the most commercially important biodegradable and biocompatible polymers, with a wide array of applications in packaging, microelectronics, and biomedical fields. Crystalline isotactic PLA (it-PLA) is produced by either ring-opening polymerization (ROP) of S,S-lactide (L-LA) to isotactic poly(L-lactide) (PLLA), or the stereoselective ROP of rac-LA by metal-based catalysts or initiators. The emerging organopolymerization of LA offers a metal-free alternative to the PLA products that are free of residual metal contaminates as desired for biomedical or microelectronic applications. The stereoselective organo-polymerization of rac-LA has been achieved by a phosphazene superbase (tBu-P2) and bulky N-heterocyclic carbenes (NHCs) at low temperatures, and enantioselective or kinetic resolution polymerization of rac-LA by chiral organic catalysts has also been made possible by a cinchona alkaloid and chiral phosphoric acids.
The current industrial PLLA production relies on the large-scale production of L-LA, which is furnished via metal (tin)-catalyzed depolymerization of a low molecular weight (MW) condensation prepolymer of L-lactic acid. However, this process affords a mixture of LA stereoisomers containing that are predominately L-LA but also a considerable amount of meso-LA as a by-product or waste that needs to be removed from the rest of the LA stream, seriously affecting the economy of the manufacturing of PLLA. In addition, as a possible feedstock-recycling pathway, thermal degradation of PLLA produced many kinds of degradation products including LA diastereomers (rac-LA/meso-LA=2/1), cyclic oligomers and their diastereomers, as well as CO2, CO, CH3CHO, and CH2═CHCOOH. Hence, racemization causes a serious problem with the feedstock recycling and the reproduction of PLLA. The optical purity of LA significantly affects the materials properties of the PLLA produced by ROP, and any incorporation of meso-LA into the PLLA product will significantly alter or deteriorate the properties of the PLLA materials such as crystallinity and biodegradation rate.
The ROP of meso-LA typically forms predominantly atactic, amorphous PLA, but the stereoselective ROP of meso-LA has led to syndiotactic PLA (st-PLA) or amorphous heterotactic PLA. The crystalline st-PLA has a melting-transition temperature (Tm) of ˜150° C., which is about 20-30° C. lower than that of it-PLA produced by either ROP of L-LA or stereoselective ROP of rac-LA, thus an inferior material. Hence, it is of great interest to be able to catalytically isomerize meso-LA to rac-LA. However, the epimerization under base-catalyzed homogeneous conditions is controlled by the equilibrium between meso-LA and rac-LA, achieving only an equilibrium mixture; citing the state-of-the-art example here, epimerization of meso-LA by sodium ethylhexanoate (0.05%) in bulk at 160° C. for 15 h afforded a mixture containing 36% L-LA+36% D-LA+28% meso-LA, plus 0.5 wt % linear oligomers (see U.S. Pat. No. 9,035,076 (Benson et al.)). Accordingly, there is a need for a solution to the challenges facing the effective utilization of meso-LA.